Assuming that you have PHP installed, and the module available for Apache, these lines need to be in httpd.conf for PHP support:

DirectoryIndex index.php index.html
<Files *.php>
SetOutputFilter PHP
SetInputFilter PHP
LimitRequestBody 10000000
</Files>
AddType application/x-httpd-php .php

Now, these lines are often in other files that are included; however, if .php pages just spew the text of the program rather than the generated content, then you probably need to add something like the above.